In a global positioning system such as the GPS (Global Positioning System) or the like, defining as a pseudorange a value obtained at a user's GPS receiver by multiplying the difference between a time of transmission of a signal such as a ranging signal (L1C/A), which is a GPS positioning signal from a GPS satellite, and a time of reception of the signal at a receiving station (monitoring point) by the speed of light, a position of the receiving station is measured on the basis of the pseudorange.
Accuracy of the pseudorange is degraded when C/No (carrier to noise power density ratio) decreases because of power reduction of the ranging signal, a clock error, an error due to the ionosphere, that due to the troposphere, noise due to multiple paths or the like, trouble of the GPS receiver itself and the like. In cases accuracy of the pseudorange is degraded, a user incorrectly determines his/her own position. Accordingly, self-position determination based on a low-accuracy pseudorange results in dangerous information for a high-speed mobile object such as an airplane.
In this respect, in usual global positioning systems, in order to remove noise due to multiple paths or the like from a measured pseudorange, smoothing processing is performed on the pseudorange. However, because such real time smoothing processing is simple one, its accuracy cannot be said to be high, and thus some portion of the noise may remain, and accordingly, there may be cases where the positioning accuracy is finally decreased.
In this respect, Japanese Patent Application Laid-Open No. 2005-249653 discloses a pseudorange evaluation system comprising a data processing device which, when an abnormal value is detected in pseudorange values after real time smoothing processing used for positioning computation, determines whether or not a pseudorange value after post-process smoothing processing measured at the same monitoring time as that of the abnormal value is normal value.
If it is found that the pseudorange was determined to be an absolute value in spite of its being a normal value, owing to the real time smoothing processing, this data processing device determines that the cause of the abnormal value detection is residual noise due to a limit of the real time smoothing processing. By this way, the cause of a decrease in pseudorange accuracy can be recognized.
Alternatively, International Publication No. WO2006-132003 discloses a GPS receiver device comprising a position correction data calculation means which calculates the position of a base station on the basis of satellite signals and thus calculates data for position correction indicating a deviation of the calculated position from the absolute position of the station, and a base station receiving-intensity measurement means which measures receiving intensity of the satellite signals at the base station.
This base station transmits to a mobile station the data for position correction calculated by the position correction data calculation means and the receiving intensity of the satellite signals measured by the base station receiving-intensity measurement means. The mobile station compares the receiving intensity of the satellite signals at the base station, measured by the base station receiving-intensity measurement means, with receiving intensity of the satellite signals at the mobile station (own station) measured by itself. By this way, recognition of a satellite signal influenced by multiple paths and correction of a measured position of a mobile station was made possible.